The HSTS header can be added to AWS ELB applications through the Application Load Balancer.
1. Locate the Application Load Balancer in AWS Control Panel
2. Find the HTTPS listener and edit it
3. Click the Attributes tab > Edit
4. Enable the Add HTTP Strict Transport Security (HSTS) Header and paste in the value max-age=2592000
5. Save Changes
https://docs.aws.amazon.com/elasticloadbalancing/latest/application/load-balancer-listeners.html